(1) Analysis of biologic products a) Description of virus morphology in influenza vaccine samples after various numbers of passages in eggs. b) Description of fibers of unknown origin in vaccine samples. (2) Research a) Studies on the effect of MHC genes in the regulation of retroviral production in mouse melanoma cells. Using immunoelectron microscopy we have found that transfection of the MHC Class I H-2Kb, but not the Class II H-2IAk gene results in the virtual elimination of two actively produced endogenous retroviruses, a budding ecotropic C-type particle and an intracellular intracisternal A-type particle (IAP). In H-2Kb-positive cells, the loss of IAP was due to a lack of IAP-specific mRNA while the loss of C-type retroviral particles was associated with deletions in their proviral DNA. b) Transfection of the MHC Class I H- 2Kb gene into cells of a pigmented subclone of the B16-F10 melanoma cell line results in the loss of melanin production. Electron microscopy showed that melanosomes, the organelles in which melanin is synthesized, were completely absent in transfected cells. Northern blot analysis showed that transfected cells lacked mRNA for the critical tyrosinase enzyme as well as mRNA for the Melanocyte Stimulating Hormone receptor. Expression of the H-2Kb gene in BL6 melanoma cells thus results in down- regulation of the entire melanogenic pathway including the inhibition of tyrosinase and MSH receptor gene expression and melanosomal biogenesis. (3) Research support. The EM Staff has provided collaborative support for a wide variety of research projects initiated by CBER scientists: a) Identification of a viral contaminant in a bioassay test system as the cause of erratic results. b) Determination of the content and homogeneity of subcellular fractions. c) Determination of the subcellular localization of viral and bacterial antigens by immunogold labeling. d) Identification of apoptotic cell death in U937 cells infected with Sendai and Newcastle Disease viruses. e) Provided morphologic evidence of loss of mitochondrial function in apoptosis.